Measurement while drilling (MWD) techniques have been developed in the field of oil and gas well drilling to provide accurate information concerning operating conditions within an oil/gas borehole and to reduce rig down-time.
Various different approaches have been developed in this field. The first and simplest is a form of earth signalling, where a length of drill pipe is considered to be a “lossy” coaxial wire extending to surface. This method relies upon the earth having a finite resistance to support an electromagnetic field and consequently, this method is limited by the particular types of rock in which the borehole is drilled.
A second approach was developed using the drill pipe as a dipole aerial, treating the system as a very low frequency (VLF) radio system. Physical constraints, particularly the wavelength of the signal compared to the limited space in a borehole, impart severe practical difficulties in this approach.
In the 1970's, mud-pulse technology was developed, which enabled details of a wide variety of borehole conditions, such as depth, temperature and pressure to be transmitted to surface. However, this method has two significant limitations. Firstly, it relies upon a column of fluid and treating this fluid as having a limited compressibility, otherwise the signal is quickly “damped” to extinction. Secondly, the data transmission rate, even in relatively incompressible fluids is limited for any significant depth of borehole.
Over the last few years, MWD has extended to logging whilst drilling (LWD), where additional sensors are located at the bottom of a drill pipe, to measure parameters indicative of the rock type being drilled. This has created a need for higher data transmission rates, which cannot be met by mud-pulse telemetry.
The most significant change in the drilling industry in recent years has been the advent of air/foam replacing conventional drilling muds in drilling techniques. This technique has advantages in particular situations, for example, where the producing formation is fragile and over-pressure from “heavy” fluid columns can cause significant damage. Also, this can cause large volumes of the expensive drilling mud to be lost in the formation, which then has to be produced out of the well before oil/gas can be recovered.
Accordingly, there has been a return to the earth signalling approach where higher data transmission rates are obtainable. In this technique, an electromagnetic field is generated downhole in a drill string. This requires an insulated gap to be provided in the string. Current techniques include coating a sub with a thin coating of a ceramic material. This method is expensive, and the coating itself is costly, difficult to apply, and susceptible to damage, particularly on thread crests, when the sub is made-up to a tool/string. Also, the coating cannot be readily inspected for integrity, and cannot be refurbished or easily replaced.
It is amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages. In particular, it is amongst the objects of embodiments of the present invention to provide an electrically insulating device, an improved electrically insulated downhole assembly and a corresponding method which can be used in improved MWD techniques.